1. Field of the Invention
The present invention relates to a piezoelectric oscillator, and relates, in particular, to a method of suppressing a current of a piezoelectric element.
2. Description of the Related Art
In recent years, following a request for a smaller size and higher performance of a mobile communication apparatus and a transmission communication apparatus, a piezoelectric oscillator such as a crystal vibrator used as a frequency control device in these apparatuses is also required to have a smaller size and higher stability. The piezoelectric oscillator has a configuration of a combined oscillation circuit including a frequency adjusting circuit and a frequency temperature compensation circuit, to work for a piezoelectric vibrator such as a crystal vibrator.
The piezoelectric vibrator is an electromechanical vibrator. As smaller amount of current flows to the piezoelectric vibrator (hereinafter referred to as a vibrator current), the piezoelectric vibrator has higher reliability in aging. FIG. 18 illustrates one example of a Colpitts oscillation circuit according to a conventional silicon transistor. A piezoelectric oscillation circuit has the following configuration. A series circuit composed of a capacitor Cb and a capacitor Ce as a part of a load capacitance is inserted and connected between a base of an oscillation transistor TR11 and the ground. A connection midpoint of the series circuit and an emitter of the oscillation transistor TR11 are connected together, and an emitter resistor Re is also connected to the connection midpoint. Furthermore, a base bias circuit composed of a resistor RB11 and a resistor RB12 is connected to the base of the oscillation transistor TR11. A series circuit of a piezoelectric vibrator Xtal and a capacitor C11 is inserted and connected between the base of the oscillation transistor TR11 and the ground. Further, a collector of the oscillation transistor TR11 and a power supply voltage Vcc line are connected together.
FIG. 19 illustrates one example of the Colpitts oscillation circuit according to the conventional silicon transistor connected in cascade. The configuration of the circuit in FIG. 19 is different from that shown in FIG. 18 in that a transistor TR12 of which base is connected to the ground is connected in cascade to the TR11. FIG. 21 illustrates an equivalent circuit when the circuits shown in FIG. 18 and FIG. 19 are in a steady oscillation. FIG. 22 illustrates an equivalent circuit in a state that the parallel connection is converted to a series connection. The vibrator current is calculated with reference to this equivalent circuit, based on the following conditions. First, as an assumption, the emitter output of the oscillation during the normal time is set as the constant voltage supply Ve, and the resistance Re and the capacitor Ce of the emitter circuit are set as the internal impedance of the power supply. As the piezoelectric vibrator oscillates in series resonance, the impedance is set to 0. Calculation expressions based on the above conditions are as follows.r1=Rπ/{1+(ω(Cb+Cπ)Rπ)2}c1=1/ω2(Cb+Cπ)Rπ·r1r2=Re/{1+(ω·Ce·Re)2}c2=1/ω2·Ce·Re·r2z=r1+1/jω·c1+r2+1/jω·c2=r1+r2+1/jω·(1/c1+1/c2)|ix|=Ve/Z=Ve/[(r1+r2)2+{1/ω·(1/c1 +1/c2)}2]1/2   (1)where    Z represents an impedance between the voltage supply Vcc end of the crystal oscillator and the ground,    r1 and r2 represent resistors based on the parallel-to-series conversion shown in FIG. 22,    c1 and c2 represent capacitors based on the parallel-to-series conversion shown in FIG. 22,    Rπ represents an input resistance of the transistor in a parallel equivalent circuit shown in FIG. 21,    Cπ represents a junction capacitance of the transistor in the parallel equivalent circuit shown in FIG. 21,    Re represents emitter additional resistance of the transistor in the parallel equivalent circuit shown in FIG. 21,    Ce represents an emitter additional capacitor of the transistor in the parallel equivalent circuit shown in FIG. 21,    ω represents an angular frequency (=2πf)    Ve represents a steady emitter output voltage,    ix represents a vibrator current, and    |ix| represents an effective value of the vibrator current.
FIG. 23 illustrates a result of obtaining by simulation the vibrator current |ix| characteristics with the capacitor Cb based on the Exp. (1). The abscissa represents the capacitance Cb between the base and the emitter, and the ordinate represents the vibrator current ix. The simulation is carried out based on the following conditions. In the equivalent circuit shown in FIG. 21, Rπ=2600Ω, Cπ=12 pF, Re=1KΩ, Ce=150 pF, Ve=2 Vrms, and F=10 MHz. FIG. 24 illustrates a result of measuring a relationship between the capacitance Cb between the base and the emitter and the vibrator current ix when Re=1KΩ, RB11=RB12=10KΩ, and the respective emitter capacitors Ce are at 150 pF, 180 pF, and 200 pF in the circuit shown in FIG. 18. The abscissa represents the capacitance Cb between the base and the emitter, and the ordinate represents the vibrator current ix. From this result, it is clear that the capacitance Cb between the base and the emitter increases in the range from 0 pF to about 100 pF. In proportion to this increase, the vibrator current ix increases. When the Cb is in a higher range from 100 pF to the above, the vibrator current ix becomes substantially constant. The result of the experiment indicates that the vibrator current ix shows a maximum value of 6500 μA.
There is another method of suppressing the increase in the vibrator current. As shown in FIG. 20, a circuit is configured with an oscillation circuit 101 composed of a Colpitts circuit, and an AGC circuit 104. Diodes 116 and 117 rectify the oscillation output, and the base current of the oscillation circuit 104 is decreased, thereby to suppress the gain. The vibrator current is suppressed as a result. According to this method, the current suppression effect is large. However, the circuit apparently becomes complex, and this circuit cannot be easily mounted on a small oscillator, which results in cost increase.
According to the conventional Colpitts oscillation circuit, the vibrator current increases along the increase in the capacitance between the base and the emitter, and there is a limit to the suppression of the vibrator current. Further, according to the method using the AGC circuit, the circuit becomes complex. Consequently, the circuit cannot be provided in a smaller size, which leads to cost increase.